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The free prostate-specific antigen test, known as the free PSA test, is used to help detect signs of prostate cancer. The test measures the level of unbound prostate-specific antigen in the blood, which can help doctors to diagnose issues like inflammation of the prostate or cancer.

Free PSA tests are often used alongside other PSA tests to confirm a diagnosis or test results. Free PSA tests can also be used instead of a biopsy if the doctor suspects prostate cancer, but a biopsy may still be needed.

In this article, we examine when a free PSA test is used, how it differs from a total PSA test, and what the results mean.

What is the difference between PSA and free PSA?

Doctors can use the prostate-specific antigen test to detect signs of prostate cancer.

Prostate-specific antigen (PSA) is a protein made mainly in the prostate. A bit of PSA is released into the bloodstream during each ejaculation, and PSA in ejaculate makes it easier for the semen to carry sperm to the fallopian tubes during conception.

PSA can have two basic states. It may be bound to another protein or float freely.

The total PSA and free PSA tests both measure levels of PSA in the blood. Results help doctors to diagnose issues like prostate cancer and inflammation of the prostate.

The tests are slightly different. A total PSA test measures all the PSA, including the antigens that are bound and those that are floating freely.

A free PSA test only measures the amount of PSA that is floating freely in the bloodstream, without being bound to a different protein.

Both tests are used to diagnose prostate issues. A person may have an increased risk of prostate cancer when they have higher levels of total PSA and lower levels of free PSA.

Understanding the free PSA test

A free PSA test is typically given after a total PSA test, to help doctors rule out cancer or to indicate how far cancer has progressed after treatment.

The test involves taking a blood sample, which is sent to a laboratory for analysis. The free PSA test is often used before a biopsy to provide initial confirmation of a doctor's diagnosis.

There are some drawbacks to using PSA testing. There may be high rates of false positives, meaning that the test can show lower levels of PSA when levels are actually normal.

In some cases, normal results from the tests do not rule out prostate cancer. Even when PSA levels are too low to indicate a problem, a person can still have prostate cancer. People may also have elevated levels of PSA without having cancer.

Because of this, PSA tests are often just one part of a complete diagnosis. If results are at all unclear, many doctors still recommend a biopsy or additional testing to help with diagnoses.

Does a person need both tests?

The total PSA test and free PSA test are often ordered together. This is because the free PSA test measures how much PSA is unbound, and this result is best understood by comparing it with how much total PSA is in the bloodstream.

Understanding the results

When total PSA levels are higher than the appropriate range, a person may have an increased risk of prostate cancer.

Doctors interpret the results based on several factors, including:

Total PSA

Total PSA ranges may vary slightly. According to a review of studies, the normal ranges for total PSA in nanograms per milliliter (ng/mL) are:

age 49 or younger: 0.0–2.5 ng/mL

age 50 to 59: 0.0–3.5 ng/mL

age 60 to 69: 0.0–4.5 ng/mL

age 70 or older: 0.0–6.5 ng/mL

When total PSA levels are higher than the appropriate range, a person has an increased risk for prostate cancer. It does not show conclusively that a person has cancer. Doctors will then compare the results of this test with results of a free PSA test.

Free PSA ratio

A high total PSA level and a low free PSA level generally indicate a risk for prostate cancer.

Free PSA ranges can vary, but in general, a higher ratio of free PSA to total PSA is healthier.

As authors of the review point out, when the ratio of free to total PSA is between 0 and 10 percent, the risk of having cancer may be as high as 56 percent. When this ratio is greater than 25 percent, this risk reduces to 8 percent.

However, these results are not conclusive evidence for or against a cancer diagnosis, and a doctor may still want to do a tissue biopsy.

Other factors

Doctors will consider additional factors, such as a person's age, ethnicity, and prostate volume. For instance, PSA levels normally rise with age, and this can affect the results of the tests.

Other factors, called PSA kinetics, may also influence how a doctor views the results in people who are currently undergoing treatment for prostate cancer.

PSA kinetics reflect how levels change over time. It can be difficult to obtain a clear picture, but kinetics can play an important role in the outlook and treatment of prostate cancer. The factors involved are PSA velocity and doubling time.

PSA velocity measures how rapidly levels are rising, and it can indicate that prostate cancer is recurring.

Doubling time is a calculation of how long it takes for PSA levels to double. A faster doubling time can suggest a more aggressive cancer, which can be a sign that the cancer is spreading.

Doctors may also recommend testing for other markers that can indicate cancer, including:

human kallikrein-related peptidase 2

prostate cancer antigen 3

[-2] proPSA

prostate health index

TMPRSS2-ERG gene fusion

Other factors that affect PSA levels

Bumping the prostate while cycling can affect PSA levels.

While PSA levels are often useful when forming a diagnosis or monitoring cancer during treatment, a number of outside factors can influence them. This why doctors take time and use different diagnostic methods before coming to a conclusion.

Other factors that can affect PSA levels include:

an enlarged prostate gland

medications that treat an enlarged prostate gland

bumping the prostate during activities like riding a bicycle, motorcycle, or horse

Free PSA or biopsy?

In some cases, free PSA tests can reduce the number of unnecessary biopsies. A test is a far less invasive procedure, and it is also quicker and cheaper than a biopsy.

However, testing and a biopsy are often both necessary to ensure a correct diagnosis. Also, results of total and free PSA tests are usually compared, and each test may help to make up for errors in the other.

Free PSA levels may be used to make a diagnosis if a person's total PSA levels are high but a biopsy shows no sign of cancer. A biopsy may miss cancer, but in many cases, the results of a free PSA test after a biopsy give a doctor confidence in a negative diagnosis.

In other cases, if PSA tests indicate that a person is at risk for prostate cancer, a biopsy is often the next step toward a diagnosis.

Takeaway

A free PSA test plays an important role in the diagnosis of prostate issues, such as cancer.

However, the results are only meaningful when compared with those of a total PSA test. Doctors often order a free PSA test after noticing that total PSA levels are high. Even then, the results do not automatically indicate a cancer risk.

Doctors may order a number of tests to diagnose problems with the prostate. In some cases, they may need to repeat tests after time has passed.

If PSA tests indicate a high risk of prostate cancer, and no other factors can explain these results, doctors often recommend a tissue biopsy.

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